1. Field of the Invention
This invention generally relates to methods and systems for inspection of a wafer or setting up an inspection process. Certain embodiments relate to detecting defects on a wafer at different focus offsets and determining if the defects are defects of an underlying layer or an uppermost layer formed on the wafer.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. Inspection has always been an important part of fabricating semiconductor devices such as integrated circuits. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the device to fail. For instance, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size has become necessary since even relatively small defects may cause unwanted aberrations in the semiconductor devices.
Inspection tools can be categorized generally according to their optical configuration. For instance, some inspection tools are configured to perform inspection using a brightfield configuration, and other inspection tools are configured to perform inspection using a darkfield configuration. In general, a brightfield configuration is used to detect light reflected specularly from a wafer. In contrast, a darkfield configuration is used to detect light scattered from a wafer. Therefore, depending on the characteristics of the wafer and the characteristics of the defects of interest, one configuration may be more suitable for inspection of the wafer than another.
The different configurations also have different constraints on their usefulness based on their performance characteristics such as sensitivity, resolution, throughput, number of nuisance defects detected, and the like. One such constraint on the usefulness of brightfield tools is the detection of defects of underlying layers on a wafer, which can dominate the inspection results and make analysis of the inspection results difficult. For instance, different inspections are typically performed on different levels of the wafer to understand the defects that occur at various process steps. Due to the transparency of the materials that are processed in some process steps, process variations and other defects at previous process levels may be detected during inspection of these materials. The detection of these process variations at previous process levels can overwhelm the resulting defect population and make separation of the current layer defect signals from the overall signals difficult. These problems have been a constant challenge to brightfield inspection.
Many different approaches have been tried to separate prior level events from current level events. Events can be generally defined as abnormalities on a wafer that may be defects. The terms “events” and “defects” are used interchangeably herein. One such approach involves inspecting the wafer after each level is formed and performing defect source analysis (DSA) of the results of both inspections. DSA generally includes analyzing inspected features and locations of defects on the same wafer at different points in time, or in the manufacturing process. Further description of DSA can be found in U.S. Pat. No. 5,991,699 to Kulkarni et al., which is incorporated by reference as if fully set forth herein. With this approach, inspection is performed at two different levels of the wafer such that the events that are common to the two processing steps (i.e., defects detected by both inspections) can be filtered out of the results for the current level.
However, the above-described method does have some drawbacks. For instance, DSA requires lot holding and inspection at prior levels that may not be critical layers in of themselves thereby resulting in lower productivity. In addition, coordinate inaccuracy between the two inspections may result in poor DSA results, lower prior level noise suppression, and/or missing current level defects. DSA also cannot be performed unless inspection results from the prior level are available.
Another approach involves using nuisance filtering algorithms to filter defects of previous levels from results of the inspection of a current level. This approach also has inherent drawbacks. For instance, the algorithms can only be used if there are distinguishable attribute differences between events of the previous level and events of the current level. Therefore, the application of this approach may be relatively limited.
Accordingly, it would be advantageous to develop methods and systems for inspection of a wafer that can be used to discriminate between defects of an underlying layer and defects of an uppermost layer formed on a wafer, that do not use results of previous inspection of the underlying layer thereby improving productivity, that are relatively insensitive to coordinate inaccuracy, and that can be used to distinguish between defects on different layers that may or may not have different attributes.